An Auckland dad was left gobsmacked after receiving a tax bill for $23,000 addressed to his 8-year-old son.
The Inland Revenue Department (IRD) have since confirmed the letter was sent by mistake.
Papakura resident George Hawkins said he couldn't believe his eyes when he opened the letter on Thursday.
His son Nikolai's name was correct, including his middle name, and was matched with the right IRD number, but Hawkins couldn't fathom how a Year 4 schoolboy could have incurred such an enormous tax bill.
All of Hawkins' children have IRD numbers so the family can receive Working for Families tax credits.
"He's not out there working," he said.
"I was really surprised. I thought 'do I have to pay them?'."
Hawkins' confusion turned to frustration and anger after trying repeatedly to get in touch with the IRD, with no success.
He got told by an automated service no one was free to deal with his request and the call was cut off.
Following questions from the Herald, an IRD staffer got in touch with Hawkins and confirmed the message had been sent to his son in error.
Hawkins said the IRD staffer told him they suspected someone put Nikolai's IRD number in by mistake when filling their tax form out online, meaning his son's information ended up linked with a stranger's tax return.
An IRD spokesman said the agency did not comment on specific cases, but confirmed this was possible, as information entered manually by a customer had to be cross-referenced and verified with existing customer information by an IRD staffer.
"It is during this verification process when some errors can regrettably occur from time to time," the spokesman said.
Customers could avoid this possibility by using their myIR account.
Hawkins said he was relieved it had been sorted out and was now curious as to whether it had happened to other people as well.
"I'm just wondering how easy it is for this to happen."
The IRD spokesman said April to June was the busiest time of the year for the IRD in terms of customers calling in.
"Occasionally our phone system simply becomes overloaded because of the volume of callers.
"When this occurs, some calls are not allowed to enter the queue because they can't be answered within a reasonable timeframe. The customer will be played an automated message advising them their call can't be answered and is then disconnected."
This was a last resort and an alternative to calling was to send a secure mail via the online myIR service, the spokesman said.